1. Field of the Invention
The present invention generally relates to a frequency discriminator for converting an input signal to a DC voltage or a DC current at the level corresponding to the frequency or average frequency of the input signal and, more particularly, to a frequency discriminator having high accuracy in frequency identification and a wide capture range. The invention further relates to application of the frequency discriminator to a heterodyne receiver for coherent lightwave communications.
2. Description of the Related Art
The coherent lightwave communications system has improved reception sensitivity by 10 to 20 dB over conventional intensity modulation/direct detection (IM/DD) and facilitates optical frequency-division multiplex. Therefore, the system is expected to be put to practical use in high speed and long distance transmission. In the heterodyne receiver for coherent lightwave communications, a local light beam is mixed with a signal light beam received and the mixed light beams are optically detected by a photodiode having a square-law detection characteristic and thereby an intermediate-frequency signal is obtained. The intermediate-frequency signal is demodulated to a baseband signal. Since the frequency of the intermediate-frequency signal corresponds to the difference between the frequency of the signal light and the frequency of the local light, if the center frequency of the signal light and/or the local light is varied by changes in ambient conditions or the like, the center frequency of the intermediate-frequency signal is also varied. A variation in the center frequency of the intermediate-frequency signal hinders normal demodulation. Therefore, in the heterodyne receiver, automatic frequency control (AFC) is generally performed to keep the center frequency of the intermediate-frequency signal constant.
In practicing AFC, a frequency discriminator is used. The frequency discriminator applied to AFC in the heterodyne receiver converts the input intermediate-frequency signal to a DC voltage at the level corresponding to the center frequency of the input intermediate-frequency signal and outputs the DC voltage. Frequency discriminators of the described type are required to have the following characteristics:
(A) That accuracy in frequency identification is high. In other words, that the output voltage level greatly varies with a change in the frequency. As the accuracy in frequency identification is increased, frequency stability in the automatic frequency control is improved.
(B) That the range of frequencies for which automatic frequency control can be achieved by a control loop (capture range) is wide.
There has so far been known a frequency discriminator, in which it is adapted such that an input signal to be subjected frequency discrimination is divided into two signals, one of the divided signals is given a predetermined delay time, and this signal is mixed with the other of the divided signals and the mixed signal is output.
In such prior art frequency discriminator, there exists a trade-off between improvement of the accuracy in the frequency identification and expansion of the capture range, namely, it is impossible to satisfy both requirements at the same time.